The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 108 |
| 5 | 600 |
| 6 | 3368 |
| 7 | 18968 |
| 8 | 106906 |
| 9 | 602532 |
| 10 | 3395402 |
| 11 | 19131460 |
| 12 | 107788900 |
| 13 | 607274848 |
[1,4,20,108,600,3368,18968,106906,602532,3395402, 19131460,107788900,607274848] |